Recently, organic photoconductive materials have been widely studied and developed so as to be used not only for electrophotographic photoreceptors (which will be also referred to merely as “photoreceptors”) but also applied to organic electro luminescent (abbreviation: EL) devices etc. Further, the electrophotographic photoreceptors using organic photoconductive material are not limited to the field of copiers, but are also used in the fields of printing plates, slide film and microfilm in which photographic technology was previously employed. Further, they are also applied to high-speed printers which use laser, light emitting diode (abbreviation: LED), cathode ray tube (abbreviation: CRT) or the like as a light source. This means that there are high and wide demands for organic photoconductive material and electrophotographic photoreceptors using it.
Conventionally, inorganic photoreceptors having photosensitive layers mainly composed of inorganic photoconductive materials such selenium, zinc oxide, cadmium and the like have been widely used as electrophotographic photoreceptors. Though the inorganic photoreceptor has basic characteristics to some degree as a photoreceptor, it involves problems such as difficulties in forming a photosensitive layer, poor plasticity, high manufacturing cost and the like. Further, inorganic photoconductive materials generally have strong toxicity, hence a severe restriction is imposed when they are handled and used for manufacturing.
In contrast, the organic photoreceptor using organic photoconductive material exhibits good performance in film-forming of photosensitive layers, is excellent in flexibility, lightweight and highly transparent. Further, since the organic photoreceptor has advantages such as that it can be readily designed so as to present excellent sensitivity for a wide wavelength range by an appropriate sensitization process, it has gradually come to play the leading role in the development of electrophotographic photoreceptors. The initial organic photoreceptors had sensitivity and durability drawbacks, but these drawbacks have been markedly improved by the development of a function-separation type electrophotographic photoreceptor in which the function of charge generation and the function of charge transport are allotted to different substances. The function-separation type photoreceptor is advantageous in presenting a wide selection of materials for charge generation material providing the function of charge generation and for charge transport material providing the function of charge transport and in enabling relatively easy fabrication of electrophotographic photoreceptors having arbitrary characteristics.
As the charge generation material to be used for such a function-separation type photoreceptor, many kinds of substances such as phthalocyanine pigments, squarylium dyes, azo pigments, perylene pigments, polycyclic quinone pigments, cyanine dyes, squaric acid dyes, pyrylium salt dyes, etc., have been investigated, and various kinds of substances having strong light resistance and high charge generation capability have been proposed.
On the other hand, as the charge transport material, various compounds such as pyrazoline compounds (c.f. Japanese Patent Publication No. Sho 52-4188), hydrazone compounds (c.f. Japanese Patent Application Laid-open No. Sho 54-150128, Japanese Patent Publication No. Sho 55-42380 and Japanese Patent Application Laid-open No. Sho 55-52063), triphenyl amine compounds (c.f. Japanese Patent Publication No. Sho 58-32372 and Japanese Patent Application Laid-open No. Hei 02-190862), and stilbene compounds (c.f. Japanese Patent Application Laid-open No. Sho 54-151955 and Japanese Patent Application Laid-open No. Sho 58-198043), are known. In recent years, pyrene derivatives, naphthalene derivatives, terphenyl derivatives (c.f. Japanese Patent Application Laid-open No. Hei 07-48324) and the like, having condensed polycyclic hydrocarbons as a central parent nucleus, have been developed.
The charge transport material is required:    (1) to be stable to light and heat;    (2) to be stable to ozone, nitrogen oxides (NOX) and nitric acid etc., which arise during corona discharge for charging the photoreceptor surface;    (3) to have a high charge transport capability;    (4) to have a high compatibility with organic solvents and binders; and    (5) to be easy to produce and inexpensive.However, the aforementioned charge transport materials satisfy part of these requirements but have yet to meet all these requirements to high levels.
Further, of the above requirements, the charge transport material is particularly required to have a high charge transport capability. For example, when a charge transport layer of the charge transport material dispersed with the binder resin forms the surface layer of the photoreceptor, the charge transport material is required to have a high charge transport capability in order to secure a high enough light response. When the photoreceptor is provided and used for a copier, laser beam printer or the like, the photoreceptor surface layer is forced to be partially scraped off by contact members such as a cleaning blade, charging roller etc. Therefore, in order to enhance the durability of the copier or laser beam printer, a robust surface layer against these contact members, that is, a high wear-resistant surface layer which is less scraped off by the contact members is demanded. If, for this purpose, the binder resin content in the surface layer or charge transport layer is made higher in order to make the surface layer strong and durable, the light response lowers. This occurs because the charge transport capability of the charge transport material is low and the charge transport material in the charge transport layer is diluted with increase of the binder resin content. As a result, the charge transport capability of the charge transport layer is further lowered resulting in poor light response. Since degradation of the light response will cause increase in the residual surface potential and the photoreceptor will be repeatedly used without its surface potential sufficiently attenuated, the surface charge in the areas to be removed by exposure to light cannot be adequately eliminated, causing degradation of image quality and other defects at an early stage. This is why a high charge transport capability is wanted for the charge transport material in order to secure high enough light response.
Further, with the recent development of electrophotographic apparatus such as digital copiers and printers into miniaturization and high-speed configuration, high-sensitive photoreceptor characteristics are demanded to deal with a high-speed configuration. Therefore, a further increased charge transport capability is demanded for the charge transport material. Further, since the time taken from exposure to development is short in a high-speed process, photoreceptors of good light response are demanded. As stated above, since the light response depends on the charge transport capability of the charge transport material, charge transport material having a high charge transport capability is wanted from this viewpoint.
As the charge transport material that meets the above requirements, enamine compounds having a higher charge mobility than the aforementioned charge transport materials have been proposed (see Japanese Patent Application Laid-open No. Hei 02-51162, Japanese Patent Application Laid-open No. Hei 06-43674, Japanese Patent Application Laid-open No. Hei 10-69107 and Japanese Patent Application Laid-open No. Hei 10-239875).
However, the photoreceptors formed with the enamine compounds disclosed in Japanese Patent Application Laid-open No. Hei 02-51162, Japanese Patent Application Laid-open No. Hei 06-43674 and Japanese Patent Application Laid-open No. Hei 10-69107 are not effective enough. On the other hand, since the enamine compound disclosed in Japanese Patent Application Laid-open No. Hei 10-239875 presents a highly symmetric configuration when a bis-butadiene substructure is introduced, the compound will exhibit markedly bad compatibility with the binder resin, hence will present detrimental effects such as partial crystallization at film forming, etc. As a countermeasure against this problem, it is possible to make improvement to some degree by replacing the substituents of the enamine structure, from a large structural unit such as an aryl group etc., to a methyl group, the minimum structural unit. However, from a viewpoint of electric characteristics (particularly, in view of hall drift mobility), an aryl group is preferred as the substituents of the enamine structure to an alkyl group; this is the problem to overcome.
Also, the photoreceptor is wanted to have reliable characteristics, presenting little variation in characteristics under various environments without lowering its sensitivity even in a low-temperature environment. However, no charge transport material which can also realize such characteristics has been obtained so far.